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Shannon Francis [email protected] 1
Ferocious Tyrannosaurs to Clucking Hens: How Chickens Evolved from Dinosaurs
Chickens are not exactly the most ferocious of creatures. They seem far more suited to clucking
and pecking about the farmyard and brooding over their clutch of eggs than to using razor sharp claws
and wits to turn some of the biggest creatures to have walked the earth into helpless prey. However,
chickens have a much grander and more vicious past than their benevolent appearances let on; one of
America's favorite barnyard friends is in fact a living dinosaur. While their bigger and badder ancestors
died off approximately 65 million years ago, there is substantial evidence that suggests that chickens, as
well as other birds, are the evolutionary offspring of a group of dinosaurs known as theropods.
Anatomical and behavioral similarities link birds to their reptilian relatives, and prove that chickens
and other birds evolved not alongside but from dinosaurs. By studying the fossils and other evidence
left behind by dinosaurs, scientists can learn more about the evolutionary process that turned creatures
such as the fearsome Tyrannosaurus Rex into the modern day chicken. Furthermore, the similarities
between living chickens and extinct dinosaurs allows paleontologists to use modern birds to make
assumptions and draw conclusions about what dinosaurs were like in the past. However, in order to
understand this particular evolutionary line, one must first understand the concept of evolution.
Charles Darwin first put forth the Theory of Evolution in his groundbreaking book The Origin
of Species. To state it simply, evolution is “the change in the gene pool of a population from generation
to generation by such process as mutation, natural selection, and genetic drift” (Dictionary.com). Even
within members of the same species, there is a great deal of genetic diversity caused both by mutation
and the recombination of genes from previous generations. This means that the genetic traits of the
species are ever-changing; this is due to the ever-changing nature of the Earth’s environments. Those
species that can adapt, be it to better obtain resources or better take advantage of their ecosystem, are
the creatures who survive to breed and create the next generation, while those who cannot simply die
off; this is the process of natural selection, one of the basic driving forces of evolution. It is through
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this process of natural selection, adaptation, and genetic mutation that a creature such as a theropod can
evolve into something as seemingly unrelated as a modern chicken.
Chickens' ancestors come from a group of dinosaurs known as coelurosaurs. These dinosaurs
were far from the popular image of giant, reptilian monsters, however; coelurosaurs were “probably
feathered, some could fly, and new evidence shows that certain coelurosaurs grew, reproduced, and
even slept like birds” (Brusatte). Following the principals of evolution explained earlier, one can
assume that the traits that chickens and other birds have in common with their coelurosaurs are not
simply coincidence, but rather positive traits that were passed down from generation to generation and
along the evolutionary line. However, coelurosaurs are not the only dinosaurs that chickens are similar
to. Coelurosaurs were part of a larger group of dinosaurs known as theropods, a group which modern
birds are placed in as well. This means that we can not only compare chickens to coelurosaurs, but also
to their evolutionary cousins such as Velociraptor and Tyrannosaurus Rex. In fact, when comparing the
anatomy of chickens and various species of theropod, one can draw many comparisons between
everything from the structure of the skeleton to the intricacies of the cardiovascular system.
When looking for similarities between theropods and chickens, one must first turn to their
bones. Not only do these two creatures share light, hollow bones, but many structural similarities as
well. One of the most prominent skeletal features that chickens and dinosaurs share is the structure of
the hips. Theropods belonged to a group of dinosaurs known as “saurichians,” or “lizard-hipped”
dinosaurs (as opposed to “ornithiscians,” or “bird-hipped” dinosaurs). Contrary to the name, birds are
not descended from ornithiscians; the hips of ornithischians only superficially resembled those of birds,
and modern birds are in fact classified as saurischians. This is due to similarities in many of the
structures in the hip, most notably the ilium. The ilium is “the uppermost and widest of the three bones
that fuse together to form each of the hipbones” (The American Heritage Science Dictionary). In most
reptiles this bone is extremely short, which means that it is unable to support anything more than
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narrow thigh muscles. Chickens and theropods, on the other hand, have much longer ilia, which allows
for larger, stronger thigh muscles that can support bipedal movement (Paul). This also results in similar
hip positioning, where the bodies of both chickens and theropods “hang down from a vertebral column
that balances, more or less horizontally like a teeter-totter, across the hip joint” (Kaiser).
In addition to similarities in the hip, theropods and chickens share many other skeletal traits,
especially in the hindlimb. Unlike sauropods and stegosaurs, which evolved straight-jointed limbs
similar to those of an elephant, theropods had large hips and slender, flexed legs that allowed them to
run at speeds similar to those of modern ground birds. This is because their knees moved in a motion
similar to that of modern birds, where the knee had to be flexed to be fully articulated (Paul).
Furthermore, theropods and chickens also share four-toed feet, where three main toes support the
animal's weight and the first toe is reversed, as well as elongated bones between the ankles and toes.
Even the structure of theropod arms and hands share similarities to the wings of a chicken;
while somewhat hard to believe, the skeletal structure of a chicken's wings does contain the remnants
of three fingers on each wing. Furthermore, paleontological evidence “indicates that three digits in the
hand of birds and maniraptorian theropods are 'I-II-III' of the ancestral five digits” (Zhou). This means
that modern chickens and the small coelurosaurs who are their closest non-avian evolutionary relatives
shared the same three digits derived from the original five digits of an ancestor further up the
evolutionary chain. Furthermore, these same, small theropods and birds share a “flattened, half-moon-
shaped bone in the wrist that limited movement of the hand, much like a similar element of the bird
wrist” (Brouchu). This means that theropods had wrists that were semi-fixated, an intermediary
between fully-flexible wrist and the fixed wrists that make up the structure of a chicken's wings.
In the past, the relationship between birds, such as chickens, and dinosaurs was heavily debated
because of the supposed lack of a crucial avian skeletal feature in theropods; “birds have a very large
set of collarbones, or clavicles, that are fused to form together the wishbone. No dinosaurs then known
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had a collarbone” (Brochu). This led scientists of the time to
believe that, instead of being direct descendants of dinosaurs,
birds had evolved alongside the beasts and were descended
from some other kind of archosaur. Although dinosaurs with
clavicles had been discovered as early as the 1920s, their
discoveries went largely unnoticed until over 40 years later. It
was not until 1964, when John Ostrom of Yale University noted the staggering amount of similarities
between theropod skeletons and bird skeletons, that the idea that birds evolved from dinosaurs was
brought back into scientific light; furthermore, the discovery of Sue (figure 1), and exquisitely
preserved Tyrannosaurus Rex, showed clear evidence of a wishbone, “the thin, slightly curved
horizontal bone between the two massive shoulder bones” (Wild Birds Unlimited).
The anatomical similarities are not limited to hard tissue, however. Under extraordinarily rare
conditions, the soft-tissue of a dinosaur will be preserved – and it is in the soft tissue that some of the
most compelling evidence for the evolutionary relationship between chickens and theropods can be
found. The first and possibly most striking example is the similarities between the respiratory systems
of birds and theropods. Birds have quite possibly the most efficient and complex breathing system of
all vertebrates. Unlike other animals such as reptiles and mammals, birds do not have dead-end lungs;
“instead, they are connected to a large complex of air sacs whose flexibility and especially volume
greatly exceed those of the lungs” (Paul). These sacs can sometimes extend as far as the pelvis and
allow most of the fresh air inhaled by the animal to bypass the lungs on its way in. After going through
the air sacs, the air is “injected through the lungs in one direction on its way out. Because this
unidirectional airflow eliminates the stale air that remains in dead-end lungs at the end of each breath
and allows the blood and airflow to work in opposite, countercurrent directions and maximize gas
exchange, the system is very efficient” (Paul). While early theropods show no evidence of air sacs,
Figure 1
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later specimens have pneumatic vertebrae and increased hinge jointing of the ribs, “indicating that they
were probably helping to ventilate the lungs by inflating and deflating air sacs” (Paul). As theropods
further evolved, their air sacs lengthened and their lungs became shorter and stiffer, as seen in modern
birds. The soft-tissue similarities of theropods and birds are not limited to the respiratory system,
either.
Most modern reptiles such as snakes, lizards, and turtles, have three-chambered hearts,
incapable of producing high blood pressure. Crocodilians have four-chambered hearts, but, like lizards,
are incapable of producing high blood pressure. Birds, on the other hand, have “fully developed, four-
chambered, double-pump hearts able to propel blood in large volumes at high pressures” (Paul). In the
past, all scientists could do was assume that dinosaur hearts were similar to those of birds, as soft tissue
very, very rarely was preserved. However, the earth-shattering discovery of a small dinosaur called
Thescelosaurus soon gave scientists more solid evidence. Its heart was preserved in an ironstone
nodule within the animal’s chest. This was the result of a very rare process where iron-bound oxygen
in the heart muscle mineralizes while in contact with groundwaters – in layman’s terms, iron in the
heart caused it to fossilize. This meant that scientists could study the structure of a dinosaur heart and
confirm that it was in fact a four-chambered heart, similar to that of birds. Furthermore, the preserved
heart gave scientists evidence that some dinosaurs (namely smaller theropods) had high metabolic rates
and were therefore warm-blooded, just as modern birds are. This would also tie into the presence of
feathers on some theropods, which would act as insulation only for warm-blooded creatures.
While many people today picture dinosaurs as great scaled beasts, the truth is that many of them
had feathers – or at least some form of precursor. Recent archaeological discoveries in China have
shown that, before Archaeopteryx, many theropods as early as the beginning of the Cretaceous period
had basic, feather-like filaments covering their bodies. The discovery of the small theropod
Sinosauropteryx was among the first; when it was discovered in 1996, its fossil clearly showed a coat
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of fine filaments covering its body. While these filaments lacked the complexity of a feather, they were
still far more advanced than reptilian scales. This rare preservation of soft filaments led scientists to
speculate that many other dinosaurs hat protofeathers that were not preserved. Another theropod,
Sinornithosaurus (a relative of Velociraptors) was also found to be covered in protofethers, and,
“although unable to fly, [it] might have been able to 'flap' its arms to catch small prey out of the air”
(Brochu). Later species, such as Caudipteryx and dinosaurs of the genus Beipiaosaurus, had more
complex, quill-like and even barbed feathers, probably used for display and insulation.
The coloring of modern chickens is even
something that was shared by their dinosaur
ancestors. Groundbreaking new research has
allowed scientists to determine the coloring
of dinosaur protofeathers by studying the
shape and density of fossilized pigmentation
organelles underneath an electron
microscope. Like modern birds, the shape of
the different organelles, called “melanosomes,” dtermines which colors they are able to produce.
Rodlike eumelanosomes produce black and grey; round phaeomelanosomes produce reddish-brown to
yellow pigments; and a lack of melanosomes produces the color white. When this technique was
applied to a fossil of the chicken-sized dinosaur Anchiornis huxleyi (figure 2), a pattern very similar to
that of a modern silver-spangled Hamburg chicken emerges on its leg feathers (Sloan). This data
concerning the coloring of a non-bird dinosaur (as many scientists classify modern birds as dinosaurs)
lead to groundbreaking revelations concerning the purpose of pre-flight feathers, as well as further
evidence of the similarities between modern chickens and their dinosaur ancestors. Dinosaur feathers
had many uses before flight was achieved, “including sexual display, territorality, et cetera.... It could
Figure 2
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also have been like modern redstarts, which use their bright wing and tail patches to scare up insects,
which [the birds] then seize in flight” (Sloan). The similarities between these dinosaur behaviors and
those of modern birds lends credence to the theory that these uses for feathers other than areal
locomotion were passed down through evolution from theropods to modern birds.
However, as strong as the similarities between chickens, and other birds, and theropods are,
their evolutionary relationship cannot be proven without a link between the two. Without this missing
link, the stunning similarities between the two were assumed to simply be a case of covergant
evolution, where similar traits evolved in two different evolutionary trees by coincidence. However, in
the 19th century, when Dinosaurs were still very poorly understood and Darwin had just published the
Origin of Species, many people still believed that both dinosaurs and birds had been created at the same
time by God. The theory of evolution was still struggling to gain footing in both public opinion and the
scientific community. By coincidence, however, 2 years after Darwin had published his theories, a
very strange fossil, seeming to be a hybrid of both bird and lizard, was discovered near Eichstätt,
Germany. It had a very dinosaurian skeletal structure, complete with a “long, bony tail and teeth [that]
were similar to those of reptiles, but the stunningly preserved feathers and light, hollow bones were
trademark features of birds” (Brusatte). It also had arms with wing-like proportions and wrists that
were partially fixed in a similar fashion to the wrists in bird wings. But perhaps most stunning of
all,the feathers of this creature were arranged in the exactly the same pattern as the feathers on modern
birds. The strange hybrid was dubbed “Archaeopteryx,” and was dubbed one of the earliest examples
of birds. However, it wasn't until Darwin's theory of evolution came to be more widely accepted that
the scientific community began to realize that the fossil of Archaeopteryx was not just a record of one
of the earliest birds, but rather a snapshot of evolution in process. The combination of bird-like and
theropod-like anatomical structures and features made Archaeopteryx not only one of the earliest
discovered examples of evolution, but also the perfect missing link between birds and dinosaurs. In
Shannon Francis [email protected] 8
more recent times, even more missing links have been found that further fill in the gaps between
dinosaurs, Archeopteryx, and birds. With such evidence creating a visual reference of the evolutionary
process between theropods and birds, their ancestral relationship is all but cemented in scientific fact.
While chickens are certainly not creatures that are expected to be seen soaring through the skies,
they are capable of flying in short bursts. They take to the air to briefly explore their surroundings,
clear obstacles, and find places to roost for the night. However, even this basic and rather primitive
form of flight is something that chickens and dinosaurs seem to lack. It is difficult to imagine
dinosaurs being capable of short bursts of flights similar to chickens, let alone true flight that many
other birds are capable of. The transition from ground-based theropods into soaring, flying birds (and
even less skilled, frantically flapping and hopping chickens) is an important part of the evolution from
dinosaurs into birds. The answer to the question as to just how flight evolved is up to some debate; the
two theories are the Arboreal Theory (“from the trees down”) and the Cursorial Theory (“from the
ground up”). The Arboreal Theory states that tree-dwelling reptiles developed a form of gliding (and
eventually flight) that allowed them to jump from tree to tree without facing the dangerous predators
that lurked on the ground, much as a modern flying squirrel does today. On the other hand, the
Cursorial Theory states that flying started with leaps and jumps in attempts to escape prey and outrun
predators, that eventually evolved into more efficient gliding and finally flight. However, the Cursorial
theory has faced much criticism due to “the problems of drag and needing to work against gravity. It is
biomechanically easier to evolve flight from gliding than from the ground…” (Kate). Furthermore,
Archaeopteryx is thought to have lacked a supracoracoideus system – the tendon that powers the
upstroke of a bird’s flight. Experiments conducted on pigeons have shown that the lack of a
functioning supracoracoideus tendon could not take off from ground level. However, there is no
evidence to suggest that Archaeopteryx had the ability to climb trees, and there is evidence of an
absence of large trees near the areas inhabited by Archaeopteryx. Due to the overall weak evidence to
Shannon Francis [email protected] 9
support the Arboreal Theory, most scientists believe that a modified version of the Cursorial Theory is
the likely origin of flight.
The closest sister groups of birds, such as Deinonychus and other dromaeosaurs, had a
sideways-flexing wrist joint used to seize prey. However, in birds, this same type of wrist joint is
essential to the production of thrust; “it would have only taken a slight adjustment of the angle of attack
of this predatory stroke to create a suitable vortex wake. By running, leaping, and a few such strokes,
extension of the time in the air, and eventually flight from the ground up, could have evolved” (Kate).
Running leaps to catch airborne prey such as insects was aided by feathered arms or wings outstretched
for balance, and the wings evolved to expand at the distal ends to increase stability when in the air.
Short flapping motions mimicking the already-present sideways-thrust provided even more airtime, and
these leaps were further extended by jumping from small heights. Eventually, through evolving more
physical and behavioral adaptations to extend time in the air, short leaps and glides evolved into full-
fledged flight.
While fossil evidence can tell us much about many aspects of Dinosaurian anatomy,
appearance, and even the development of flight, it is extremely difficult to determine how dinosaurs
behaved using physical evidence alone. This is where their relationship with chickens comes in handy.
Because the natural selection process favors positive behavioral traits as well as anatomical ones, so it
can be assumed that chickens share both anatomical and behavioral similarities with dinosaurs. By
looking to chickens as well as other birds, scientists can make educated guesses about how chickens'
reptilian ancestors behaved hundreds of millions of years ago.
One of the most widespread views of chickens in popular culture is that of the brooding hen.
Chickens are often depicted as loving mothers attentively caring for their chicks. Dinosaurs, though
related to chickens, are afforded a much different image in popular culture; they are viewed as simple-
minded, violent creatures who, like many modern reptiles, abandon their offspring as eggs and lack all
Shannon Francis [email protected] 10
maternal instincts. Even though fossils of adult dinosaurs had been found on nests of fossilized eggs, it
was assumed for many years that these dinosaurs were simply stealing other species' eggs to eat. This
even resulted in some dinosaurs being labeled as egg-stealers, such as Oviraptor, which was discovered
in Mongolia on top of a nest of eggs that were believed to belong to a herbivore known as
Protoceratops. However, once an embryo that had been preserved in one of the eggs was examined, it
was determined that the eggs belonged not to Protoceratops but to Oviraptor; “poor Oviraptor,
maligned as an egg stealer, was apparently brooding its own nest as subsequent discoveries by
American Museum and Sino-Canadian expeditions have confirmed” (Meier). Subsequent fossils were
even found “huddled over its fossilized eggs in a posture identical to that of a modern bird” (Brochu).
In light of this evidence as well as the similarities to modern birds, it can be deduced that at least some
species of dinosaurs actively cared for their eggs. However, evidence has shown that, like chickens,
dinosaurs were rather attentive mothers as well.
When examining the nest of a duck-billed dinosaur called Maiasaura, John R. Horner's team of
scientist discovered two groups of young Maisaura fossils, the first group being newborn hatchings and
the second group being substantially older. Upon looking at the fossils under the found that “the ends
of the limb bones had been cartilaginous at the time of death. Such immature limbs were too weak for
the young animals to have run about on their own. Similar growth patterns occur in the nest-bound
young of some birds” (Meier). By comparing similar anatomical aspects of the young dinosaurs and
young birds, scientists can assume that, like chickens, dinosaurs had to care for their vulnerable young
until they were mature enough to fend for themselves. This brooding and maternal behavior further
links dinosaurs and birds such as chickens together.
Despite their less than ferocious image in popular culture, chickens are in fact very close
evolutionary relatives of theropod dinosaurs that lived millions of years ago. The anatomical and even
behavioral similarities between the two offer very strong evidence for this relationship, and the
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discovery of missing links such as Archaeopteryx has shown that chickens, as well as other birds, did in
fact evolve from dinosaurs. Because of this close relationship, modern chickens can be used to learn
more about dinosaurs by comparing behaviors, anatomy, and even appearance. Especially when one
takes the latest scientific research into account and realizes that, instead of cold-hearted, scaled, simple-
minded beasts, some types of dinosaurs were found to be feathered, fluffy, hen-like creatures with
clutches of needy chicks nipping at their heels, their relationship with chickens becomes even more
apparent. With that in mind, chickens are not simply fluffy, egg-producing birds found in farmyards
across America; they are something much more vicious and amazing than many people realize. They
are dinosaurs.
Shannon Francis [email protected] 12
Works Cited
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to Dinosaurs. Ed. Michael K. Brett-Surman. San Francisco: Fog City, 2002. Print.
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Darwin, Charles. The Origin of Species. Oxford: Oxford UP, 1996. Print.
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<http://www.helium.com/items/1410877-similarities-between-dinosaurs-and-birds>.
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Mayr, Ernst. What Evolution Is. New York: Basic, 2001. Print.
Sloan, Chris. "True-Color Dinosaur Revealed: First Full-Body Rendering." National
Geographic. 4 Feb. 2010. Web. 01 Nov. 2011.
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